Methods of producing zinc-doped gallium phosphide



July 23, 1968 w. WESTERVELD ET AL 3,394,035

METHODS OF PRODUCING ZINC-DOPED GALLIUM PHOSPHIDE Filed Aug. 15, 1965 INVENTORS WILLEM WESTERVELD WILHELMUS I? BE GRAAF v BY K iM/L Emmi United States Patent ABSTRACT OF THE DISCLOSURE A method of making zinc-doped gallium phosphide crystals useful in semiconductor devices, such as an injection radiation source. The method involves providing a melt of gallium and phosphorus with zinc as the dopant in the absence of oxygen but further containing tin or germanium, which acts to promote the incorporation of zinc in the gallium phosphide crystals. The gallium is present in large excess. Upon cooling, the desired crystals are separated from the excess gallium present.

The invention relates to an injection radiation source having a body of gallium phosphide containing zinc in solid solution and an electrode from where excess charge carriers can be injected into the zinc-doped gallium phosphide body, so that in the recombination process of electrons and holes in the body radiation can be produced, and relates furthermore to a method of producing zinc doped gallium phosphide by separation from a melt consisting mainly of gallium and phosphorus and furthermore zinc. The recombination need not be a direct combination of an electron in the conduction band with a hole in the valence band, it may also be performed stepwise, for example by the capture of an electron from the conduction band or a hole from the valence band in an intermediate level, after which they are recombined with a hole and an electron respectively, while during at least one of the steps radiation is produced. Instead of two steps the recombination may be performed in three or more steps.

Such a radiation source may be employed for many purposes, for example as a lamp, in electronic, optical or electro-optical devices, having at least one electro-luminescent part for example signal transmission systems with electroluminescent cells and photocells, in image intensifiers with photo-electric and electro-luminescent layers, in opto-electronic transistors and in so-called lasers, having an injection radiation source in which stimulated coherent radiation is produced.

The radiation may be produced in the semi-conductor material for example by the passage of current through a p-n, p-i-n, or metal-semiconductor junction.

It is known that zinc-doped gallium phosphide is a particularly suitable material for use in an injection radiation source in which radiation is produced in the visible region of the spectrum with a maximum at about 7000 A. and 5750 A. The yield may be very high.

It has been found, however, that in bodies obtained from a melt of gallium, phosphorus and zinc, at least at room temperature, injection luminescence could often not be produced, even when the crystals were transparent. \It has furthermore been found that the luminescence properties did appear when the melt had been contaminated by a small quantity of oxygen. The conclusion was therefore made that a satisfactory injection luminescence required the presence of zinc and oxygen in the gallium phosphide.

It is however, difficult to dose oxygen in a reproduceice able manner in the melt. The melt contains furthermore constituents reacting readily with oxygen. Solid gallium oxide may be formed in the melt. The particles thereof may operate as crystallising seeds, so that the formation of generally desired monocrystals of reasonable size is counteracted. Moreover, such particles may from inclusions in the gallium phosphide bodies obtained, which might absorb part of the radiation, when such a body is employed in a radiation source.

The invention has for its object inter alia to provide zinc-doped gallium phosphide suitable for use in an injection radiation source, which does not display said disadvantages. It has now been found that gallium phosphide obtained from an oxygen-free melt of gallium, phosphorus and zinc does not contain detectable quantities of zinc, while such a melt, which furthermore contains a small quantity of oxygen, can provide gallium phosphide containing zinc in quantities which can be readily detected spectrographically. Therefore, it is possible that the addition of oxygen has the function of enabling the incorporation of zinc in the crystal lattice of gallium phosphide. It has now been found that instead of oxygen also the presence of other elements permit of obtaining gallium phosphide with zinc doping, suitable for use in an injection radiation source.

According to a first aspect of the invention, an injection radiation source of the kind set forth is characterized in that the zinc-doped gallium phosphide of at least that part of the body into which the excess charge carriers can be injected from the electrode, contains, in addition, tin and/ or germanium. In order to obtain a high yield of radiation use is preferably made of a zinc content of at least 0.0005 by weight. With a zinc content in excess of 0.01% by weight the intensity of the radiation emanating from the gallium phosphide is considerably reduced by internal absorption so that a less high zinc concentration is preferred.

The sum of the atomic concentrations of germanium and tinthe concentration of only one of these elements being taken into account if the other of the two is lackingis preferably lower than the atomic concentration of zinc.

According to a second aspect of the invention a method of producing zinc-doped gallium phosphide by separation from a melt containing mainly gallium and phosphorus and furthermore zinc is characterized in that the melt contains, in addition, tin and/ or germanium. In the presence of one or of both last-mentioned elements in the melt zinc apears to be incorporated in the gallium phosphide without the need for oxygen to be present. Gallium phosphide doped with zinc, as stated above, may be used in injection radiation sources, but the method according to the second aspect of the invention is not restricted to the production of material for this use; if desired, the material produced by said method may also 'be used, in principle, for other semi-conductor devices for example diodes and transistors. The term oxygenfree as used in the claims is intended to denote an atmosphere substantially equivalent to that obtained by exhaustion and back filling as described in the working example hereinafter.

The atomic quantity of gallium in the melt is preferably chosen higher than the sum of the atomic quantities of the other elements in the melt. This involves that it is not necessary to carry out the formation of the melt and the separation of gallium phosphide therefrom at extremely high temperatures, while gallium phosphide crystals with a substantially constant zinc concentration may be obtained therefrom without appreciable concentration gradients. The atomic quantity of gallium in the melt is preferably at least twice the atomic quantity of phosphorus therein. In practice, it is preferred to prepare a melt having an atomic quantity of gallium at least five times and at the most twenty times the atomic quantity of phosphorus in the melt. The zinc content in the melt preferably amounts to at least 0.001% by Weight and at the most 0.2% by weight and if no germanium is added to the material to be melted the tin content is preferably at least 0.001% and at the most 1% by weight. The melt is preferably cooled gradually down from a temperature of at least 900 C.

The invention will be described with reference to an embodiment and the accompanying drawing, which shows a vertical sectional view of the radiation source.

For the manufacture of zinc-doped gallium phosphide 6.7 gs. of gallium phosphide, 1 mg. of tin, 10 mgs. of zinc and 20 mgs. of gallium are introduced into a quartz ampulla. The quartz ampulla is exhausted and then filled with hydrogen of a pressure of about /3 atmosphere, the ampulla being then sealed. Subsequently, the ampulla is heated in a high-frequency furnace for three hours at 1220 C. and then gradually cooled, the temeprature dropping within a period of time of about 2 hours from 1220 C. to about 800 C. After cooling to room temperature the ampulla is opened and the ingot is removed. The zinc-doped gallium phosphide crystals formed can be separated from the solidified gallium by dissolving the gallium in 36% by weight of hydrochloric acid. The crystals obtained may be sorted to size and, if desired, larger crystals may be divided into smaller monocrystal bodies of the desired dimensions.

There now follows an example of the production of a light source from a body manufactured in the manner described above from zinc-doped gallium phosphide. The body 1 of this example (see the figure) has a length of about 4 mms., a width of about 2 mms. and a thickness of about 0.3 mm. Onto one side of the body 1 are alloyed two metal pellets. One pellet has a diameter of 400 to 500,12 and consists of an alloy of gold and zinc (4% by weight of zinc). The second pellet has a diameter of about 300, and consists of tin. The two pellets are alloyed at a temperature of 1000 C. to 1100 C. The gold-andzinc pellet constitutes an ohmic contact 2 with the body 1 and the tin pellet constitutes a rectifying contact 3 with the body 1. The rectifying contact 3 constitutes the injecting electrode of the light source. Tin-clad copper wires 4 and 5 are soldered to the contacts. A voltage of 2.2 v. is applied in the forward direction between the contacts, so that in the body 1, in the proximity of the contact 3, there is produced an intensive orange-red radiation with a maximum of about 7000 A. and a maximum of about 5750 A.

What is claimed is:

1. A method of producing zinc-doped gallium phosphide crystals, comprising forming a melt consisting essentially of gallium and phosphorus as the main constituents, a minor amount of zinc as a dopant, and in addition a minor amount of at least One element selected from the group consisting of tin and germanium, the gallium being present in the melt in an atomic quantity which exceeds the sum of the atomic quantities of all the other elements, cooling said melt to form said doped crystals, and separating said crystals from the excess gallium present, said melt-forming and cooling steps being effected in an oxygenfree atmosphere.

2. A method as set forth in claim 1 wherein the atomic quantity of the gallium in the melt is betwewen two times and twenty times the antomic quantity of the phosphorus present in the melt.

3. A method as set forth in claim 2 wherein the zinc content in the melt is between 0.001% and 0.2% by weight.

4. A method as set forth in claim 3 wherein the melt contains tin, and the tin content is between 0.001% and 1% by weight.

5. A method as set forth in claim 4 wherein the crystals are formed by gradually cooling the melt down from a temperature of at least 900 C.

References Cited UNITED STATES PATENTS 12/1966 La-rnorte 313108 12/1966 Biard et a1. 3l3108 OTHER REFERENCES TOBIAS E. LEVOW, Primary Examiner.

J. COOPER, Assistant Examiner. 

